Arrays of surface-bound binding agents may be used to detect the presence of particular targets, e.g., biopolymers, in solution. The surface-bound probes may be oligonucleotides, peptides, polypeptides, proteins, antibodies or other molecules capable of binding with target molecules in solution. Such binding interactions are the basis for many of the methods and devices used in a variety of different fields, e.g., genomics (in sequencing by hybridization, SNP detection, differential gene expression analysis, identification of novel genes, gene mapping, finger printing, etc.) and proteomics.
One representative array assay method involves biopolymeric probes immobilized in an array on a substrate, such as a glass substrate or the like. A solution containing analytes that bind with the attached probes is placed in contact with the array substrate, covered with another substrate such as a coverslip or the like to form an assay area and placed in an environmentally controlled chamber such as an incubator or the like. Usually, the targets in the solution bind to the complementary probes on the substrate to form a binding complex. The pattern of binding by target molecules to biopolymer probe features or spots on the substrate produces a pattern on the surface of the substrate and provides desired information about the sample. In certain instances, the target molecules are labeled with a detectable tag such as a fluorescent tag or chemiluminescent tag. The resultant binding interaction or complexes of binding pairs are then detected and read or interrogated, for example by optical means, although other methods may also be used. For example, laser light may be used to excite fluorescent tags, generating a signal only in those spots on the biochip that have a target molecule and thus a fluorescent tag bound to a probe molecule. This pattern may then be digitally scanned for computer analysis.
As such, optical scanners play an important role in many array based applications. Optical scanners act like a large field fluorescence microscope in which the fluorescent pattern caused by binding of labeled molecules on the array surface is scanned. In this way, a laser induced fluorescence scanner provides for analyzing large numbers of different target molecules of interest, e.g., genes/mutations/alleles, in a biological sample.
For each pixel of a scan, a detector (e.g., photodetector such as a photomultiplier tube) may detect light emitted from the surface of a microarray, and output an analog signal line that changes in amplitude according to the amount of emitted light entering the detector. This analog signal is usually sampled and digitized using an analog-to-digital converter (A/D converter) and integrated using a digital signal processor (DSP) to provide data, e.g., a numerical evaluation of the brightness of the pixel. This data is usually stored and analyzed at a later date.
During data analysis, signals for pixels of each feature of an array are integrated to provide an evaluation of the level of target bound to each feature of the array. A critical step in the analysis of raw array data, therefore, is identifying which pixels represent features (i.e., which features are “feature pixels”), and which pixels do not represent features (and are therefore represent “background” pixels). Accordingly, there is a need for improved methods for identifying feature pixels, in order to increase the accuracy and reliability of processed array data.
The present invention meets this, and other, needs.
Literature of interest includes: U.S. Pat. No. 6,674,882, published U.S. patent applications: 20030168579, 20030165871, 20040064264, 20040023224, 20040021911, 20030203371 and 20030168579; and Cheung et al., Nature Genetics 1999, 21: 15-19.